Optical phase conjugation is a technique for reversing both the direction of propagation and the overall phase factor of an incoming wave. More precisely, optical wave conjugation has been described as a non-linear optical process for generating an output polarization that radiates a time-reversal optical field with a spatial phase profile proportional to the complex conjugate of an input optical field.
A thorough exposition of the principles of phase conjugate optics is found in a book entitled "Phase Conjugate Optics" authored by Jun-Ichi Sakai, published by McGraw-Hill, Inc., New York (1992), that is part of the Advanced Science and Technology Series. This book teaches a variety of techniques for achieving optical phase conjugation and of apparatus for utilizing this phenomenon and such teaching is incorporated herein by reference.
The principal use of optical phase conjugation is its use as a mirror to provide a reflected beam that retraces its original path independent of the angle of incidence on the conjugator providing the phase conjugation. The utility of the phase conjugation process largely stems from the fact that the useful properties of the conjugate beam ideally are not at all affected by the interposition of a distorting medium between the source of original beam and the reflector used to form the phase conjugate beam. This makes it possible through optical phase conjugation to double pass a high quality optical beam through a poor quality optical system ideally without loss of beam quality.
This ability of phase conjugation to correct phase aberration in a transmission path adapts a phase-conjugate mirror (PCM) for use as one reflector in an optical resonator for use with optical laser oscillators and in multiple pass optical laser amplifiers. In this application the PCM can compensate for any intracavity phase aberration. Introducing an aberration in the optical resonator does not change the nature of the output beam exiting at the conventional mirror of the resonator. The quality of the output beam from the resonator is entirely controlled by the quality of the conventional mirror.
Moreover, a unique characteristic of an optical resonator that includes a PCM, to be described hereinafter as a phase conjugate reflector PCR, is that it does not exhibit frequency drift and mode hopping as the distance between the conventional mirror and the PCM changes. This happens because the phase accumulated in travel in one direction between the two mirrors is cancelled in travel between the two mirrors in the opposite direction.
There is known a variety of ways for achieving optical phase conjugation. Of particular interest for the present invention is degenerate four wave-mixing. Here, two pump light waves of the same wavelength but from opposite directions are passed through an appropriate medium having a third-order optical non-linearity. Then when a third (probe) wave of the same wavelength is passed through the same medium but in a different direction, there is formed a phase conjugate wave (the fourth wave) of same frequency but opposite in direction to the third wave.
A problem that has limited the usefulness of an arrangement of this kind for generating optical phase conjugation is the inefficiency of the process with the result that only a small fraction of the energies of the three input waves becomes available as the energy of the fourth newly-generated beam, the useful optical phase conjugate. The process would be considerably more useful if there were available structures more efficient for generating the optical phase conjugate beam.